1. Field of the Invention
The invention concerns a coherence microscope and a method of operating such a microscope.
2. Description of the Related Art
In conventional microscopy problems are involved in the sharp representation of spatially extensive objects. Image sharpness is adversely affected due to blurred contributions of object regions above and below the focal plane. Different apparatuses and methods of imaging spatially extensive objects have therefore been developed.
One method with which sharp images can be obtained from spatially extensive objects utilises a confocal optical system for the imaging procedure. The concept of imaging by means of a confocal optical system is described for example in U.S. Pat. No. 3,013,467. That concept is based on the fact that a light source in point form is made available for example by means of an aperture member with a small hole (pinhole), the light of that light source being focussed on to a point on the specimen. The light reflected by that point on the specimen is in turn focussed on to a point which represents an image of the point of the specimen. Arranged at the location of that image is a second pinhole, behind which is disposed a detector for detecting the reflected light. Only light originating from the focal plane is projected on to a point at the location of the second aperture member and can pass through the pinhole. Light which has been reflected at the specimen in front of or behind the focal plane in contrast forms a disk at the location of the second aperture member. Such light therefore cannot pass through the pinhole so that essentially only light from the focal point reaches the detector. Accordingly images of even spatially extensive objects can be sharply produced by a confocal imaging procedure as contributions from regions of the object which are above or below the focal plane are not involved in the imaging procedure. Slot apertures can also be used instead of pinholes. In that case the image of the light source on the specimen is in the form of a line and the image of the light reflected by the specimen appears as a line on the same or a further slot aperture.
Confocal microscopes, that is to say microscopes based on confocal imaging, are used for example as laser scan microscopes (LSM) in particular in biology, material science and medical diagnostics. In terms of intraoperative diagnostics the particular challenge on the corresponding microscopic scanning method is to be fast, of high resolution and compatible for use in an endoscope. Methods of that kind can be used to effect optical biopsies for tumor detection for example in the gastrointestinal tract.
A confocal laser scan microscope in conjunction with an endoscope is for example the laser scan microendoscope described in Y S Sabharwal et al ‘Slit Scanning Confocal Microendoscope for High Resolution In-Vivo Imaging’, Appl Opt 34, pages 7133-7144 (1999). That instrument produces the image of a laser beam by means of a confocal aperture member on the specimen, scans the specimen with the laser beam two-dimensionally (laterally) and receives the stray light reflected by the specimen. In order to record a spatial, that is to say three-dimensional image, two-dimensional planes are scanned at various depths. The depth of the plane to be recorded is adjusted by displacement of the focal plane of the microscope in the specimen. The result of that method is a so-called z-stack of two-dimensional images. The scans are either implemented manually (with a high degree of inaccuracy and a lack of reproducibility) or by means of miniaturised focusing devices which must satisfy high demands in terms of accuracy and reproducibility and which in addition are to be only of a small size. The mechanical demands on such focusing devices are very high. Laser scan microendoscopes have therefore not yet become commercial products.
Longitudinal resolution of the laser scan microendoscope is determined by the confocal optical system. Confocal imaging by means of an aperture member provides that only stray light from a longitudinally closely delimited region in respect of depth impinges on the detector. The depthwise extent of that region and thus the longitudinal resolution of the microscope depends on the opening in the aperture member, that is to say for example the hole of the aperture diaphragm, and in practice reaches values of typically less than 10 μm. Better levels of longitudinal resolution, that is to say more narrowly delimited depthwise regions, are possible by further closure of the aperture opening, which however is linked to a high degree of light loss. Disadvantages of the laser scan microendoscope are the long scanning time which is necessary to record a z-stack and a low level of optical sensitivity. Optical sensitivity is adversely affected by the generally low degree of transmission of the optical fiber bundle of the laser scan microendoscope and troublesome reflections at the optical surfaces.
An alternative for imaging spatially extensive objects which is not based on the principle of confocal imaging is the apparatus described in DE 199 29 406 for performing optical coherence tomography (OCT). It includes a light source which emits substantially incoherent light and a beam producing device for producing a measurement light beam and a reference light beam which is coherent with respect to a reference time relative to the measurement light beam, from the incoherent light. The specimen is irradiated with the measurement light beam. The light reflected by the specimen is picked up and spatially superimposed with the reference light beam. Because of the incoherence in respect of time of the radiation, interference phenomena occur in the superimposition operation only with substantially identical optical travel lengths of the measurement light beam and the reference light beam. Therefore different optical travel lengths for the reference light beam result in interference phenomena with measurement light reflected by the object at different depths. Thus the different optical travel lengths of the reference light beam can be related to the depth at which reflection of the measurement light took place in the specimen in order to obtain a depth profile of the specimen.
A disadvantage of the apparatuses set forth is that they cannot be used to implement an optical biopsy in the desired manner.
Therefore the object of the invention is to provide an apparatus with which an optical biopsy can be implemented in a manner which is advantageous in comparison with the state of the art. A further object of the invention is to provide a method of operating such an apparatus.